I really wish I could like this game, but as someone who's finished JC3, it is just a downgrade in almost every way.
The gameplay loop is rather tedious, instead of liberating settlements to progress and doing challenges for upgrades, you now do boring, repetitive missions. Chaos objects don't matter anymore, they simply respawn and their only purpose is to fill a bar which allows you to deploy squads so you can advance to the next region.
The grapple and tethers don't have that strength to them anymore. You don't have grenades and pistols anymore, instead you have weapon attachments, and while these can be cool, it feels somewhat restrictive. Oh, also the mines are gone too. While the game offers some new gadgets, they're niche and not as useful. Pretty much everything (including Rico) is a damage sponge now. The explosions feel underwhelming as well.
The story (I watched a Youtube video because I'm NOT going through the whole game for that)... it's eh. Feels like it takes itself too seriously despite not being great. Compare that to Just Cause 3's where it was rather simple but still enjoyable.
The ui is simply horrible. It's messy, annoying to navigate... Seriously, what we're they thinking...?
As for positives... The soundtrack is good, the map is beautiful, and the weather can be fun. Not enough to make this game good, sadly. All in all, might be an okay game if you buy it on sale and never played the previous Just Cause.

Just Cause 4 is an improvement over JC3 in many areas - the graphics are much better, the jet pack is more fun to play with, vehicle controls are better as well, but all of that does not matter, because somewhere among those all improvements one thing went missing - the fun factor, which somehow evaporated into oblivion.

The conquest of the island is now depending on highlighting the regions on the map and having 'squads' numbers - as in, smash things to get squads, and then click on the area and puff - done - area conquered. Boring, especially that on top of that you have the battles happening on the borders, which as far as i know are there just to exist - take part or not - it doesn't matter for unlocking the region.

Because of the above, smashing enemy bases again feels superficial. In JC3 you had to 'defeat' the base to conquer it, here just fly above, cause some chaos and fly away, because once you will conquer the region all the bases will be yours anyway.

tl;dr - JC4 is a technical improvement, but it is simply not as fun.

An incredible sandbox ruined by terrible gameplay design.